1. Field
The present invention relates to video image processing, and more particularly, to reducing flicker in a video image sequence.
2. Background Information
FIG. 1 is a schematic diagram illustrating an arrangement for a typical video conferencing system. In this diagram, a user/subject is positioned in front of a camera 110 while looking at a computer monitor 120. The area or room is illuminated by a light source 130. Camera 110 is designed to reduce flicker due to light source 130. This may be accomplished by setting the aperture time of the camera to capture a whole number of illumination cycles for light source 130. In this context, aperture time refers to the length of time during which the camera is sensitive to incident light. Illumination cycle refers to the multiplicative inverse of the frequency of the variation of the intensity of the light source. This may also be accomplished by setting the camera frame rate such that the frame exposure starts at approximately the same point in the illumination cycle for each frame.
For the situation illustrated in FIG. 1, computer monitor 120 acts as a second light source. The refresh rate on the computer monitor depends on many variables and, quite possibly, does not correspond to one of the frequencies handled by the camera using the previously described methods. Due to this frequency mismatch, some frames, such as in an image sequence displaying the scene illustrated in FIG. 1, may get more illumination than other frames, resulting in flickering in the video sequence that may be annoying to a human observer. A need, therefore, exists for a technique to reduce flicker for video image sequences displaying situations or scenes involving a second light source, such as a computer monitor, for example.
Briefly, in accordance with one embodiment of the invention, a method of reducing flicker in a video image sequence includes the following. Successive video images are temporally filtered. The filter length is adjusted so as to reduce the amount of filtering across temporal discontinuities in the video image sequence.
Briefly, in accordance with another embodiment of the invention, a method of reducing flicker in a video image sequence includes the following. Successive video images are temporally filtered. The filter length is dynamically adjusted.